could be physics, could be chemistry, could be both. everything from altitude to humidity to old baking powder to uneven oven temperature can cause a cake to collapse.

can you provide more information about the recipe so we can help you troubleshoot? and have you checked your oven temperature with a thermometer to confirm that it's calibrated properly and heating evenly?

Is it possible it was meant to? Was there a photograph which showed a non-sunken cake? I ask because I have a recipe for a prune and armagnac cake which is flourless and intended to be damp and 'puddingy' rather than light and spongy. Just a thought!